Hybrid Magneto-Responsive Composites Made from Recyclable Components: Tunable Electrical Properties Under Magnetic and Mechanical Fields

Abstract

This study presents the fabrication and characterization of hybrid magneto-responsive composites (hMRCs), composed of recyclable components: magnetite microparticles (MMPs) as fillers, lard as a natural binding matrix, and cotton fabric for structural reinforcement. MMPs are obtained by in-house plasma-synthesis, a sustainable, efficient, and highly tunable method for producing high-performance MMPs. hMRCs are integrated into flat capacitors, and their electrical capacitance (C), resistance (R), dielectric permittivity (${ϵ}^{\prime }$), and electrical conductivity ($\sigma$) are investigated under a static magnetic field, uniform force field, and an alternating electric field. The experimental results reveal that the electrical properties of hMRCs are dependent on the volume fractions of MMPs and microfibers in the fabric, as well as the applied magnetic flux density (B) and compression forces (F). C shows an increase with both B and F, while R decreases due to improved conductive pathways formed by alignment of MMPs. $\sigma$ is found to be highly tunable, with increases of up to 300% under combined field effects. In the same conditions, C increases up to 75%, and R decreases up to 80%. Thus, by employing plasma-synthesized MMPs, and commercially available recyclable lard and cotton fabrics, this study demonstrates an eco-friendly, low-cost approach to designing multifunctional smart materials. The tunable electrical properties of hMRCs open new possibilities for adaptive sensors, energy storage devices, and magnetoelectric transducers.

1. Introduction

The increasing demand for smart multifunctional materials with adaptable physical properties has driven intense research in sensor technologies, flexible electronics, and adaptive energy storage systems [1,2,3,4]. Among these, magneto-responsive materials are particularly promising due to their ability to modify mechanical and electrical properties in response to external stimuli, offering diverse applications in biomedical sensors [5], soft robotics [6], and active rheology control of cementitious materials [7].

Conventional magnetorheological (MR) suspensions [8,9,10,11,12] and electrorheological (ER) fluids [13,14,15] have been widely investigated for their field-dependent behavior. Despite significant advancements in field-responsive materials, several critical challenges remain in designing materials that simultaneously achieve mechanical adaptability, electrical tunability, and environmental sustainability. Traditional MR and ER suspensions are fluid-based, and introduces issues related to long-term stability, sedimentation, and the need for specialized containment systems [16,17,18]. These factors significantly limit their deployment in wearable electronics, flexible devices, and smart mechanical systems, where sustained performance is essential. Furthermore, existing MR and ER materials typically respond to either magnetic or electric fields [19,20,21,22]. However, there are few exceptions successfully combining both multifield responsiveness and structural stability within a single material system [23,24,25,26,27].

Additionally, most suspensions are based on synthetic polymers and complex chemical binders. This leads to environmental concerns regarding disposal and long-term ecological impact. The embedded microparticles, which determine the suspension’s responsiveness to external fields, are often produced through either chemical [28,29] or physico-chemical synthesis methods [30,31]. Chemical synthesis, although capable of creating dual-functional microparticles, is often hindered by the use of complex processes and environmentally hazardous reagents. Consequently, there has been a shift toward more sustainable alternatives, such as plasma-based physical synthesis [32,33,34,35] or green synthesis using waste natural materials [36,37]. These methods provide a scalable and eco-friendly approach for producing magnetite microparticles (MMPs) from recyclable materials. These include onion, potato, tea, maringa and gaharu leafs, or carbon steel waste rods.

This study aims to overcome the environmental and technological challenges associated with traditional particle synthesis methods. Here, we develop multifunctional, stable and low-cost hybrid magneto-responsive composites (hMRCs) composed of recyclable components. MMPs are synthesized through a plasma-based process that recycles carbon steel rods [38] used in civil constructions, reducing material waste and environmental impact. The composites use lard and cotton fabric as the matrix materials, both of which are commercially available and recyclable. Lard provides a stable, hydrophobic medium that reduces particle sedimentation and agglomeration due to its high viscosity and natural fat composition [39]. Its semi-solid state ensures even distribution of MMPs. When the mixture of lard and MMPs is soaked into cotton fabric, the MMPs are uniformly absorbed, creating magnetically responsive textiles with potential applications in wearable sensors, electromagnetic shielding, and protective gear. The lard also temporarily binds the particles to the fibers, enhancing particle adhesion and enabling further functionalization.

The primary objective is to investigate how these hMRCs respond to static magnetic fields and uniform compression, particularly in terms of their dielectric and electrical properties. The study also seeks to demonstrate the application of these hybrid materials in smart capacitors, evaluating their potential as field-responsive mechanical deformation sensors with adjustable sensitivity. By exploring the relationship between material composition, external fields, and electrical behavior, this work contributes to the development of sustainable, multifunctional materials for adaptive devices. These findings have potential applications in automotive sensors, wearable electronics, soft robotics, and energy storage systems, where tunable material properties are essential for performance optimization.

The remainder of this paper is organized as follows. Section 2 describes the materials and methods used to synthesize and characterize the hMRCs. Section 3 details the experimental results, focusing on the electrical properties of the fabricated capacitors and their responses to varying magnetic and mechanical field conditions. Section 4 presents an in-depth discussion of the results, emphasizing the physical mechanisms governing field-induced property changes. Finally, Section 5 provides a summary of the key findings and their implications for future applications in smart and adaptive materials.

2. Materials and Methods

2.1. Materials

For the fabrication of hMRCs, the following materials are used: MMPs (see details in Appendix A), lard (from Elit, Alba Iulia, Romania), and cotton fabric. Besides environmental friendliness, lard was chosen as the matrix material due to its high viscosity at room temperature (about 24 °C), which helps reduce sedimentation of MMPs. In addition, lard behaves as a resilient semi-solid under storage in sealed conditions and maintains stable physical properties at this temperature.

The electrical properties of MMPs, lard and fabric are measured at a temperature of 24 °C using the flat capacitor (FC) method [25]. The results are summarized in

Table 1. C, dielectric loss coefficient (D), relative dielectric permittivity (${ϵ}^{\prime }$), and dielectric loss factor (${ϵ}^{\prime \prime }={ϵ}^{\prime }D$) of MMPs, lard, and cotton fabric.

Component	C, $\left(\mathbf{pF}\right)$	D	${\mathit{ϵ}}^{\prime }$	${\mathit{ϵ}}^{\prime \prime }$
MMPs	45.50	0.1007	16.4255	1.6540
Lard	17.65	0.0209	6.3717	0.1332
Cotton fabric	19.22	0.1783	1.4473	0.2581

and show that all electrical properties are dependent on the nature of the materials used. This dependency arises from the formation of conductive nano-micro layers [23] by MMPs, between which oxides act as dielectric materials. These networks of microcapacitors result in an equivalent capacitance (C) and ${ϵ}^{\prime }$ up to 2.58 times higher than that of lard.

However, the value of ${\epsilon }^{\prime }$ for the cotton fabric is much lower compared to that of MMPs and lard. This difference is due to the presence of air in the empty spaces between the fibers of the fabric. The table also shows that the ${\epsilon }^{\prime \prime }$ of MMPs is approximately one order of magnitude higher than the values for both lard and cotton fabric. In the latter case, the value of ${\epsilon }^{\prime \prime }$ is determined by the moisture present in the cotton microfibers.

2.2. Preparation of MMPs + Lard Mixture and hMRCs

The specified quantities of MMPs and lard listed in

Table 2. Volumes and volume fractions of lard and MMPs, in the lard + MMPs mixture (Figure 1a).

Vlard (cm3)	VMMPs (cm3)	${\mathbf{\Phi }}_{\mathbf{lard}}^{\mathbf{V}}$ (% vol.)	${\mathbf{\Phi }}_{\mathbf{MMPs}}^{\mathbf{V}}$ (% vol.)
6	4	60	40

are introduced in a Berzelius cylinder and a mixture is obtained (Figure 1a). Then, the mixture is homogenized at 220–250 °C for 500 s, followed by cooling to room temperature.

Afterwards, two identical cotton fabric pieces are cut at dimensions 30 mm × 30 mm × 0.6 mm (Figure 1b) and then soaked in MMPs + lard mixture to create hMRCs samples (Figure 1c), denoted hMR${\mathrm{C}}_1$ and hMR${\mathrm{C}}_2$. Volumes and volume fractions of these components are provided in

Table 3. Volumes, volume fractions of lard, MMPs, and microfibers used in preparation of hMR${\mathrm{C}}_1$ and hMR${\mathrm{C}}_2$. $Ms$—saturation magnetization.

	Vlard (cm3)	VMMPs (cm3)	Vf (cm3)	${\mathbf{\Phi }}_{\mathbf{lard}}$ (% vol.)	${\mathbf{\Phi }}_{\mathbf{MMPs}}$ (% vol.)	${\mathbf{\Phi }}_{\mathbf{f}}$ (% vol.)	$\mathbf{Ms}$ ($\frac{{\mathit{A}\mathit{m}}^2}{\mathit{K}\mathit{g}}$)
hMR${\mathrm{C}}_1$	0.20	0.14	0.14	42	29	29	9.68
hMR${\mathrm{C}}_2$	0.31	0.28	0.14	42	39	19	13.12

. The volume ratios of lard, MMPs, and CF used in hMR${\mathrm{C}}_1$ and hMR${\mathrm{C}}_2$ were selected based on preliminary tests to ensure both structural uniformity and effective field responsiveness. The baseline composition (hMR${\mathrm{C}}_1$) offers good mechanical integrity and electrical sensitivity, while the increased MMPs fraction in hMR${\mathrm{C}}_2$ allows us to evaluate the effect of higher magnetic filler content on the tunability of electrical properties. In both cases, the volume of the lard was kept constant to facilitate comparison.

2.3. Structural Properties of hMRCs

The crystalline structure, phase, and morphology of hMR${\mathrm{C}}_1$ and hMR${\mathrm{C}}_2$ are studied using XRD and SEM techniques. XRD analysis confirmed the cubic magnetite phase in both hMR${\mathrm{C}}_1$ and hMR${\mathrm{C}}_2$ (Figure 2a). Differences in peak intensities suggest variations in crystallite size (∼3 μm for MMPs and MMPs with lard) and phase interactions. SEM data (Figure 2b) show the presence of MMPs with dimensions ranging from a few micrometers up to a few hundred micrometers stuck to the cotton microfibers. This is facilitated by the presence of lard, which covers MMPs (Appendix B).

2.4. Magnetic Properties of hMRCs

The magnetization curve of MMPs ($M{s}_{\mathrm{P}}$; Figure 3) is obtained using an experimental setup described in Ref. [40]. The magnetization curve for MMPs + lard mixture ($M{s}_{\mathrm{PL}}$; Figure 3) is obtained using the relation ${\mu }_{0}M{s}_{\mathrm{PL}}={\mu }_0{\mathsf{\Phi }}_{\mathrm{P}}^{\mathrm{V}}M{s}_{\mathrm{P}}$ [41], where ${\mathsf{\Phi }}_{\mathrm{P}}^{\mathrm{V}}$ is given in Table 2. The magnetization curves for hMR${\mathrm{C}}_1$ and hMR${\mathrm{C}}_2$ ($M{s}_{{\mathrm{hMRC}}_1}$ and $M{s}_{{\mathrm{hMRC}}_2}$; Figure 3), are obtained using a similar relation, i.e., ${\mu }_{0}M{s}_{{\mathrm{hMRC}}_i}={\mu }_0{\mathsf{\Phi }}_{{\mathrm{P}}_i}M{s}_{\mathrm{PL}}$, where ${\mathsf{\Phi }}_{{\mathrm{P}}_i}$ is the volume fraction of MMPs in ${\mathrm{hMRC}}_i$ ($i=1,2$; Table 3). The MMPs + lard mixture, which lacks the cotton matrix, displays magnetization levels between those of the hMRCs and MMPs. The lower magnetization of hMRCs (vs. MMPs) is due to the cotton matrix reducing particle alignment.

These magnetization properties play a critical role in the practical applications of hMRCs. For instance, the ability to adjust magnetization through matrix composition and magnetite content allows for tuning the sensitivity and responsiveness of mechanical sensors, such as strain or deformation sensors. The reduced coupling in fabric-embedded suspensions enhances stability under varying mechanical conditions, making them suitable for field-responsive materials in applications with precise control of rheological and magneto-mechanical behavior.

2.5. Manufacturing of FC

The manufacturing of FC is carried out using a strati-textolite (STP) with a thickness of 0.6 mm, WR-Type 611 (from Rademacher, Rhede, Germany) and hMRCs. On one side of the STP, there is a thin copper layer. From the STP board, four pieces are cut, each with dimensions 30 mm × 30 mm (Figure 4a). Two copper conductors were soldered to each STP using a standard tin-lead (Sn60Pb40) soldering alloy to ensure stable electrical contact with the measuring system. F${\mathrm{C}}_1$ and F${\mathrm{C}}_2$ are fabricated by sandwiching hMR${\mathrm{C}}_1$, and, respectively, hMR${\mathrm{C}}_2$ between the copper-coated plates (Figure 4b), with insulation tape applied (Figure 4c). Appendix C provides additional details on the components of FC.

2.6. Experimental Setup for Measuring the Electrical Properties of FC

The setup depicted in Figure 5 comprises an electromagnet, between the poles of which FCs are alternately inserted. The electromagnet is powered by a DC source (model RXN-3020D, manufactured by HAOXIN, Weifang, China). The magnetic flux density B between the north (N) and south (S) poles is regulated by adjusting the electric current supplied to the electromagnet coil (pos. 2) through the DC source. A Hall probe (h; Gaussmeter DX-102, model DX-102, from DexingMagnet, Xiamen, China) was used to measure B. The capacitors and h are secured by mechanical tensioning. This is achieved by vertically applying compression forces, in steps of 0.5 N within the interval from 0 to 2 N. These forces were applied using a non-magnetic shaft (pos. 3 in Figure 5) with a diameter of 8 mm. A turntable was fixed to the upper end of the shaft (pos. 4 in Figure 5) to enable controlled manual adjustment of the applied force, while a rigid plate was fixed to the lower end of the shaft (pos. 5 in Figure 5). This plate transmitted the force uniformly to the upper capacitor plate, which, together with the bottom plate, confined the hMRC sample. The capacitor plates, each 0.8 mm thick are made of FR4 epoxy resin reinforced with glass fiber, and have a compressive strength of 548 MPa (as indicated in the technical characteristics of the product). Thus, they are sufficiently rigid to ensure uniform pressure distribution over the entire sample surface without deformation. This setup ensured that the applied force acted uniformly across the entire area of the composite.

C and equivalent resistance (R) are measured using an RLC bridge Br (model CHY-41R, made in Hsinchu City, Taiwan). These measurements are taken while the capacitors are subjected to an alternating electric field of frequency $f=1$ kHz, superimposed with a magnetic field and uniformly applied compression forces F. The measured values are recorded after stabilization, i.e., after a time interval $t=20$ s following the application of B and F on the capacitors.

To ensure statistical reliability, each measurement of capacitance and resistance at fixed values of B and F was repeated five times over intervals of one hour. All measurements were conducted under controlled laboratory conditions at 24 °C and relative humidity of 66%. The reported values represent the mean of these measurements, and standard deviations are provided to indicate reproducibility. In all cases, standard deviations were below 2% of the mean, confirming good consistency across repeated trials.

3. Electrical Properties of FC

The measurements of electrical properties for MMPs, lard, and cotton fabric, were carried out using the FC with copper-coated plates (described in Section 2.5) of common area $S=6.25\times {10}^{-4}$ ${\mathrm{m}}^2$ (see also Appendix D). The distance between the plates was set to $h_0=2\times {10}^{-3}$ m for MMPs and lard. For CF, the separation was determined by the thickness of the material, approximately $0.6\times {10}^{-3}$ m. These values are consistently used below in calculating the electrical capacitance, resistance, and dielectric permittivity. All measurements were conducted at a controlled temperature of 24 °C. This temperature was maintained throughout the sample preparation and electrical characterization stages to ensure consistency, particularly given the temperature sensitivity of lard’s viscosity.

3.1. Electrical Capacitance and Resistance

C and R are measured in a static magnetic field superimposed on the static compressive force field. The static magnetic field is increased with a step of 10 mT, up to the maximum value of 150 mT. The variation of capacitance with B for different values of F, i.e., the function $C=C{\left(B\right)}_{\mathrm{F}}$ is shown in Figure 6. Similarly, resistance $R=R{\left(B\right)}_{\mathrm{F}}$ is shown in Figure 7.

The capacitance measurements reveal a clear dependence on both B and F. For the former case, the dependence is quasi-linear. For F${\mathrm{C}}_1$, at $B=0$ mT, C increases from 79 pF with no force to 90 pF when $F=2$ N. At $B=150$ mT, the capacitance rises to 97.2 pF without force and reaches 110 pF at $F=2$ N. Similarly, for F${\mathrm{C}}_2$, C starts at 103 pF with $F=0$ N and $B=0$ mT, increasing to 129 pF at $F=2$ N. At 150 mT, C ranges from 139 pF ($F=0$ N) to 171 pF ($F=2$ N). This demonstrates a higher overall capacitance and sensitivity for F${\mathrm{C}}_2$ due to its increased magnetite content.

The combined effects of B and F enhance polarization within the suspensions. These changes suggest that by tuning the magnetic and mechanical parameters, the dielectric response of the capacitors can be precisely controlled. This tunability is advantageous for mechanical deformation sensors, where sensitivity to small force changes is critical. For instance, in F${\mathrm{C}}_2$, the capacitance increases by approximately 30% with the application of a 2 N force at $B=0$ mT, compared to about 19% in F${\mathrm{C}}_1$.

The resistance measurements for the capacitors demonstrate a strong dependence on both B and applied compression force. Resistance decreases quasi-linearly with increasing magnetic field for both suspensions, indicating that the alignment of magnetite particles under the magnetic field enhances conductive pathways within the hybrid matrix. This effect is further amplified by compression forces, which improve particle contact and reduce insulating voids within the structure. For example, in F${\mathrm{C}}_1$, R drops from 6.156 MΩ at $B=0$ mT and $F=0$ N to 4.425 MΩ at $F=2$ N. Similarly, in F${\mathrm{C}}_2$, R decreases from 4.278 MΩ to 2.182 MΩ under the same conditions.

Comparing the two capacitors, F${\mathrm{C}}_2$ exhibits lower resistance across all field conditions due to the higher volume fraction of magnetite in hMR${\mathrm{C}}_2$. The increased concentration of conductive particles reduces the effective resistivity of the material by enhancing the density of current pathways. This behavior also suggests that F${\mathrm{C}}_2$ is more responsive to both mechanical and magnetic field inputs, as the conductive network becomes more stable and efficient under combined field conditions.

3.2. Relative Dielectric Permittivity

From the formula of the parallel-plate capacitor, the relative dielectric permittivity ${ϵ}^{\prime }$ is obtained as ${ϵ}^{\prime }=h_{0}C/({ϵ}_{0}S)$, where $h_0$ is the distance between the plates of the capacitor, ${ϵ}_0=8.854$ pF/m is the vacuum permittivity, and S is the area of the common surface of the plates of the capacitor. Thus, ${ϵ}^{\prime }$ can be written as:

$${ϵ}^{\prime }\simeq 0.361{\mathrm{pF}}^{-1}\times C\left(\mathrm{pF}\right).$$

(1)

For MMPs, lard, and cotton fabric, they are quasi-constant, and the values are reported in Table 1 (together with the dielectric loss factor ${ϵ}^{\prime \prime }$).

For the capacitors with cotton fabric, we have $h_0=6\times {10}^{-4}$ m and $S=9\times {10}^{-4}$ ${\mathrm{m}}^2$, and thus:

$${ϵ}^{\prime }\simeq 0.0753{\mathrm{pF}}^{-1}\times C\left(\mathrm{pF}\right).$$

(2)

This shows that for the capacitors used, ${ϵ}^{\prime }$ has identical shapes, with values shifted vertically downward by the corresponding coefficient. Thus, they also increase with both B and applied F due to enhanced polarization. These results indicate that magnetic and mechanical fields strongly influence the dielectric response of the suspensions.

3.3. Electrical Conductivity

Taking into account the expression of electrical conductivity ($\sigma$) of a linear resistor, for hMRCs obtained here, it can be written as:

$$\sigma =h_0/\left(RS\right).$$

(3)

Using the same numerical values as in Section 3.1, Equation (3) becomes:

$$\sigma =0.67\left(m^{-1}\right)/R\left(\mathrm{M}\mathsf{\Omega }\right)$$

(4)

The results are plotted in Figure 8 and show that $\sigma$ has a strong dependence on both B and F. $\sigma$ increases with B due to improved alignment of magnetite particles. As above, compression forces further augment this effect by reducing insulating voids and increasing particle-particle contact efficiency, thereby improving the overall conductivity.

A comparison between F${\mathrm{C}}_1$ and F${\mathrm{C}}_2$ highlights the impact of material composition on electrical properties. F${\mathrm{C}}_2$, with a higher MMPs content, exhibits significantly greater conductivity than F${\mathrm{C}}_1$ across all field conditions.

3.4. Quantification of the Contribution of Magnetic and Compression Fields on C, R, and $\sigma$

The effects of magnetic and compression field on C, R and $\sigma$ of hMRCs are quantified by the relation:

$$\alpha =\left({\displaystyle \frac{\Lambda {\left(B\right)}_{\mathrm{F}}}{{\Lambda }_0}}-1\right)\times 100,$$

(5)

where $\Lambda$ is $C{\left(B\right)}_{\mathrm{F}}$, $R{\left(B\right)}_{\mathrm{F}}$, or $\sigma {\left(B\right)}_{\mathrm{F}}$ from Figure 6, Figure 7 and Figure 8 and ${\Lambda }_0$ are the corresponding value when neither magnetic nor compression fields are applied. Note that the numerical values and coefficients in Equations (1)–(5) are specific to the geometry and dimensions of the capacitor used in this study (plate area, spacing, and dielectric thickness).

The quantification of the capacitance increase in capacitors F${\mathrm{C}}_1$ (Figure 9a) and F${\mathrm{C}}_2$ (Figure 9d) highlights the influence of both B and F. As the magnetic and compression fields increase, the C rises significantly, indicating enhanced polarization within the dielectric medium. F${\mathrm{C}}_2$, with a higher magnetite content, shows a more pronounced increase in C compared to F${\mathrm{C}}_1$.

The quantification of the resistance reduction for capacitors F${\mathrm{C}}_1$ (Figure 9b) and F${\mathrm{C}}_2$ (Figure 9e) complements the capacitance analysis, highlighting the influence of both B and F on $\sigma$. For example, at $B=0$ mT and $F=2$ N, F${\mathrm{C}}_1$ shows a resistance reduction of approximately 21%, while F${\mathrm{C}}_2$ exhibits a more substantial reduction of over 35%. At higher B (e.g., 150 mT), the reductions reach 45.2% and 78.2% for F${\mathrm{C}}_1$ and F${\mathrm{C}}_2$, respectively. These reductions correspond to $\sigma$ increases of around 90% for F${\mathrm{C}}_1$ (Figure 9c) and over 300% for F${\mathrm{C}}_2$ (Figure 9f), highlighting the significant impact of the combined magnetic and mechanical fields on enhancing material conductivity. Unlike conventional MR suspensions, which exhibit little to no change in electrical properties under a magnetic field, these changes indicate that hMRCs can function as a field-responsive dielectric material, where both charge distribution and conductive pathways are modulated by external magnetic and mechanical stimuli.

These observations correlate with the capacitance data, where both polarization and conductivity improve under field-induced particle alignment and compression [12,42]. The higher MMPs content in hMR${\mathrm{C}}_2$ results in a more pronounced reduction in resistance, consistent with its larger capacitance increase under similar conditions.

4. Discussions

4.1. Scalability and Process Optimization of Plasma-Synthesized MMPs

The feasibility of scaling up plasma-based synthesis is a crucial consideration for industrial adoption [35,38]. The method used in this study, which involves plasma arc pulverization of carbon steel rods [38], offers several advantages over conventional chemical synthesis routes, particularly in terms of sustainability and material recycling. One of the practical advantages is its ability to rapidly produce MMPs from recycled materials. These particles exhibit sufficient functional quality for composite fabrication without requiring extensive post-processing or chemical purification. This is particularly beneficial in industrial applications where large-scale production of magnetically responsive materials is required, such as in adaptive sensors, flexible electronics, and magnetoelectric transducers [43]. However, compared to conventional chemical precipitation or sol-gel methods, the plasma process requires a high initial energy input [44] to maintain the plasma arc, which could be a limiting factor for widespread commercial use.

Thus, to enhance scalability and process efficiency, alternative synthesis environments can be considered. One approach involves obtaining microparticles in an inert environment, such as argon, which prevents unwanted oxidation but does not actively reduce F${\mathrm{e}}^{3+}$ species. A more effective method could involve using a mixture of argon with reducing gases such as hydrogen or ammonia, which would help protect the MMPs from oxidation and stabilize the magnetite phase. Additionally, collection of microparticles in protective liquid matrices, such as mineral oils combined with inert or reducing gas environments, could further prevent oxidation and allow for immediate integration into liquid-based magnetorheological fluids or suspensions. These modifications could significantly improve the efficiency of plasma synthesis while maintaining high product quality, making the technique more viable for industrial applications.

The integration of these protective environments into the plasma-based synthesis process can improve the stability and yield of MMPs while simultaneously enhancing the scalability of the technique. By further optimizing plasma arc parameters, such as discharge current and feed rate, it may be possible to reduce energy consumption while maintaining high production yields. The adaptability of the plasma approach to different gas environments and collection methods underscores its potential as a scalable and sustainable alternative to traditional magnetite synthesis techniques.

4.2. Comparison with Conventional ER/MR Materials and Limitations

A significant advantage of hMRCs over traditional MR and ER suspensions is their ability to simultaneously respond to both magnetic and mechanical fields while maintaining structural stability. Conventional MR and ER fluids are widely used in adaptive damping systems, vibration control devices, and active rheology applications, where an external magnetic or electric field induces viscosity changes. However, these suspensions typically face two major challenges: (i) sedimentation instability due to density mismatches between the magnetic/dielectric particles and the carrier fluid and (ii) limited tunability in response to external fields [45]. MR fluids, for instance, are primarily responsive to magnetic fields, whereas ER fluids require high external voltages (often exceeding 3 kV/mm), making them energy-intensive and less practical for certain applications.

In contrast, the hMRCs obtained here offer improved structural stability due to their solid-state composition, where MMPs are embedded within a natural lard matrix and reinforced with cotton fabric. This design mitigates sedimentation issues commonly associated with MR suspensions and eliminates the need for specialized containment systems. Furthermore, the ability of hMRCs to modulate their electrical properties (capacitance, resistance, and conductivity) under simultaneous magnetic and mechanical stimuli distinguishes them from conventional MR/ER materials, which typically exhibit limited electrical property variations under field exposure.

The strong increase in conductivity (up to 300%; Figure 9c,f) and capacitance (up to 60%; Figure 9a,d) is coupled with a strong decrease of resistance (up to 82%; Figure 9b,e). These field-induced changes reveal that the electrical behavior of hMRCs is highly sensitive to the external environment and material composition. The observed trends are physically consistent and arise from the same underlying mechanisms. The increase in capacitance is attributed to enhanced polarization within the composite matrix, driven by the alignment of MMPs and improved interparticle contact under compression. These same structural rearrangements reduce the average interparticle spacing, forming more efficient conductive networks, which leads to an increase in electrical conductivity and a corresponding decrease in resistance. The inverse relationship between resistance and conductivity, is clearly reflected in the experimental results. Notably, hMR${\mathrm{C}}_2$, with a higher MMP content, exhibits a more pronounced response due to the greater density of particles available to form conductive and polarizable pathways.

The differences observed between hMR${\mathrm{C}}_1$ and hMR${\mathrm{C}}_2$ also highlight the critical role of composite composition in tuning performance. The increased MMPs content in hMR${\mathrm{C}}_2$ enhances field-induced alignment and conductive network formation, resulting in a more pronounced response to magnetic and mechanical stimuli. This composition–property relationship underscores the importance of optimizing filler loading to balance responsiveness with mechanical integrity and processing ease. These findings suggest that the electrical behavior of hMRCs can be finely tuned through controlled variation of magnetite concentration.

The strong field-induced increases in electrical conductivity and capacitance observed in hMRCs offer distinct practical advantages over conventional MR and ER systems. Increased conductivity enables their use as adaptive resistors and field-tunable electrical interconnects in flexible electronics, while enhanced capacitance provides greater charge storage capacity for tunable capacitors and magnetically responsive filters. Unlike conventional suspensions, which require high voltages (ER) or exhibit limited dielectric tunability (MR), hMRCs achieve significant electrical modulation under moderate magnetic and mechanical fields in a stable, solid-state configuration. This makes them well-suited for integration into compact, energy-efficient smart systems such as wearable sensors, magnetoelectric transducers, and soft robotic interfaces.

To further highlight the advantages of the developed hMRCs,

Table 4. Comparison of field-induced electrical responses in hMRCs and related MR/ER-based composites. MO—mineral oil, SO—silicone oil, $\overline{d}$—average diameter. The values of $\mathsf{\Phi }$, B, and F have been selected such that they give the largest increase/decrease of capacitance/resistance.

Study	Matrix	Filler	$\overline{\mathit{d}}$, ($\mathsf{\mu }$m)	$\mathbf{\Phi }$, (vol.%)	B, (mT)	F, (N)	Property
This work	Lard + CF	Magnetite	∼40.0	39	150	3.0	C: +65%
							R: −78%
							$\sigma$: +300%
Ref. [46]	SO	Fe2O3	∼0.94	40	400	0.0	C: +33%
							R: −72%
							$\sigma$: -%
Ref. [47]	CF	Magnetite	∼0.01	4.4	400	8.0	C: +16%
							R: −20%
							$\sigma$: +17.0%
Ref. [48]	MO + CF	Magnetite	∼0.01	6.5	0.00	9.0	C: -
							R: −76%
							$\sigma$: +620%

presents a quantitative comparison between the results obtained in this study and those reported in related literature. The comparison focuses on the percentage variations in capacitance and resistance under similar magnetic field and compression conditions. Notably, the hMRCs developed here show a capacitance increase of up to 65% and a resistance reduction of up to 78%, which are among the highest values reported. The increase in electrical conductivity by 300% is also among the highest reported. In contrast, conventional MR/ER systems based on mineral oil, silicone oil, or low-volume-fraction fillers typically achieve more modest responses, with capacitance increases generally below 35% [46,47] and resistance reductions under 78% [48]. The superior performance of hMR${\mathrm{C}}_2$ is attributed to the high filler volume fraction and the synergistic effects of compression and magnetic alignment. These data demonstrate that hMRCs exhibit enhanced field responsiveness in both dielectric and resistive behavior, underscoring their potential for use in smart sensing and adaptive electronics.

Beyond structural reorganization, several electronic transport mechanisms also contribute to the conductivity enhancement. Field-controlled dielectric breakdown within the lard matrix may occur, wherein applied fields induce polarization of molecular components, reducing resistivity in localized regions. Furthermore, magnetostriction-induced microstructural rearrangements may lead to dynamic changes in particle distribution, enhancing connectivity. In nanoscale junctions, quantum tunneling effects could also play a role, particularly under strong compression, where the narrowing of insulating barriers facilitates electron transport. The combination of these factors, i.e., particle alignment, contact resistance suppression, dielectric modifications, and tunneling contributions, explains the highly tunable electrical response of hMRCs, making them a promising class of materials for adaptive electronic components and magnetoelectric applications.

From an application perspective, the ability of hMRCs to function as mechanical deformation sensors, tunable capacitors, and adaptive resistors expands their potential use beyond traditional MR/ER applications. While MR suspensions are mainly limited to mechanical damping and actuation, and ER suspensions require high electric fields, hMRCs provide a field-controlled tunable dielectric response, making them suitable for magnetoelectric transducers, wearable electronics, and electromagnetic shielding. These multifunctional properties place hMRCs in a unique category of smart materials that bridge the gap between MR/ER fluids and solid-state adaptive electronic materials.

It is also worth noting that the compressive forces applied in this study were relatively modest and uniformly distributed across the sample area. Given the rigidity of the capacitor plates and the composite thickness (2 mm), significant macroscopic deformation is unlikely. Therefore, the observed changes in capacitance and resistance are attributed primarily to internal microstructural rearrangements [12,23]—such as the alignment and densification of MMPs—rather than to bulk geometric deformation.

4.3. Future Directions for Large-Scale Implementations

To bridge also the gap between laboratory-scale development and industrial application, future research should focus on refining the plasma-based synthesis method to further enhance its industrial scalability. This includes optimizing the plasma arc parameters to reduce energy consumption while maintaining high production yields and investigating alternative methods for controlling particle size distribution. The incorporation of protective environments should be systematically studied to determine their effects on particle stability and magnetic properties. Additionally, long-term mechanical and electrical stability testing under real-world conditions will be necessary to validate the practical applicability of these materials in commercial devices.

By addressing these challenges, plasma-based synthesis could become a key enabling technology for the large-scale production of advanced magneto-responsive composites. Further collaborations with industrial partners will be essential to optimize manufacturing conditions and to explore integration strategies for these composites in existing production lines. The potential for waste material utilization and sustainable material design makes this approach highly relevant for industries seeking eco-friendly solutions for high-performance functional materials.

5. Conclusions

This study successfully developed hMRCs using recyclable cotton fabric, lard, and MMPs synthesized from recycled carbon steel rods through an argon plasma process. hMRCs are used as dielectric materials between rigid textolite plates and the entire assembly is enclosed with self-adhesive surgical tape. This configuration in the form of a capacitor isolates the composite from ambient air and moisture, minimizing the risk of oxidation or microbial degradation. The electrical properties are systematically investigated using the obtained capacitors in an alternating electric field (f = 1 kHz), a static magnetic field, and uniform compression forces, at room temperature.

The experimental results demonstrate that the relative dielectric permittivity and electrical conductivity of hMRCs can be effectively controlled by adjusting the volume fraction of MMPs. Additionally, the application of static magnetic fields and compressive forces significantly influences the capacitive and resistive responses of the capacitors. The findings indicate that these devices can function as mechanical sensors with tunable sensitivity, where the degree of mechanical deformation detection can be modulated via an external magnetic field.

Furthermore, the ability to fine-tune dielectric permittivity and electrical conductivity at low compression forces highlights the potential of these materials for applications in adaptive sensing, energy storage, and magnetoelectric transducers. By employing sustainable and low-cost materials, this work contributes to the development of environmentally friendly multifunctional materials. While the current study focuses on the electrical tunability of hMRCs under magnetic and mechanical fields, future work will focus on scaling up production and integrating hMRCs into real-world devices for industrial testing and commercialization. This involves investigations of long-term stability of the composites, including the effects of temperature, time, and biological factors on their structural and electrical properties.